1. Field of the Invention
The present invention relates to bulk acoustic wave resonators, and more particularly, to bulk acoustic wave resonators that use bulk acoustic waves (BAW).
2. Description of the Related Art
In conventional art, it is proposed that a bulk acoustic wave resonator (which may also be referred to as “BAW resonator”) is provided with a movable section for frequency modulation.
For example, Japanese Unexamined Patent Application Publication No. 2010-516213 discloses, as illustrated in the cross sectional view of FIG. 11, a structure in which a switching element 2 is fabricated by MEMS technology to push and pull a contact pad 8 to and from an upper surface electrode 9 of a BAW resonator formed on a substrate 1. When the contact pad 8 is away from the upper surface electrode 9, an electrode thickness is relatively thin. Thus, a resonance frequency and an antiresonance frequency of the resonator are relatively high. When the contact pad 8 is in contact with the upper surface electrode 9, the electrode thickness is relatively thick. Thus, the resonance frequency and the antiresonance frequency of the resonator are relatively low.
Japanese Unexamined Patent Application Publication No. 2008-005443 discloses a structure in which a resonator body is provided with a tuning layer thereon, and the thickness of the tuning layer is varied by applying a voltage across electrodes that sandwich the tuning layer to shift a resonant frequency of a resonator.
Japanese Unexamined Patent Application Publication No. 2005-528010 discloses a structure in which electrodes are provided below a resonator with an air gap in between, and a beam portion of the resonator is bended by applying a voltage across those electrodes to shift a resonant frequency of the resonator.
In the case of Japanese Unexamined Patent Application Publication No. 2010-516213, the movable section is formed above the BAW resonator that has a certain thickness. Thus, the structure is complex, and the level of difficulty required for fabrication process is high. In particular, the level of difficulty required for the fabrication process increases when the movable section is driven with an electrostatic force.
In the case of Japanese Unexamined Patent Application Publication No. 2008-005443, the thickness of the tuning layer that is an insulating elastic body is only partially varied. Thus, even if the frequency is shifted at all, the amount of such a shift would be considerably small.
In the case of Japanese Unexamined Patent Application Publication No. 2005-528010, there is no change in the thickness of a piezoelectric film or the thickness of an electrode in a vibration region through which the vibration propagates, and only stress of the vibration region changes. Thus, even if the frequency is shifted at all, the amount of such a shift would be considerably small.